But when a lease cannot be obtained, the booting is not completed. Is it possible to leave DHCP enabled but, in case a DHCP lease is not achieved, still complete the boot (so that a new network configuration can be performed)?

This is not a reason for not completing the boot process. Maybe it gets long pauses when services (like mail) are waiting for dns resolver to timeout. In this case if you wait enough time the system will boot. Disable the boot splash screen or switch to console with ALT-F1 to see the booting messages.
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forcefsckApr 7 '11 at 12:15

What distribution is that, and what networking-related settings did you tune?
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GillesApr 7 '11 at 21:35

@Gilles Looks like Debian-based, probably Ubuntu. Still, should not be a reason for it to not boot, maybe just take longer.
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penguin359Apr 7 '11 at 23:19

Not Ubuntu, nor Debian. It is a customized busybox.
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michelemarconApr 8 '11 at 9:39